Chemical blowing agent

ABSTRACT

The present invention relates to a composition comprising a blowing agent and an activator characterised in that the blowing agent and activator have been micronised together rather than separately as hitherto; to methods for making the composition and to the use of the compositions in expanding polymers.

The present invention relates to a blowing agent composition, methodsfor making it and expandable compositions containing it.

Blowing agents which decompose upon heating to give off a gas have beenused for many years in the expansion of thermoplastic polymers. However,the decomposition temperature may be too high for a given polymer and/orthe gas may be evolved over a comparatively wide temperature range.Therefore, an activator for the blowing agent is often mixed with theblowing agent; see for example U.S. Pat. Nos. 3,305,496 and 3,511,787and Spanish Pat. No. 444996 which describe the use of heavy metal,alkali-metal or alkaline-earth-metal salts or a special form of reactivesilica as activators in azodicarbonamide blowing agents. Whilst theactivators and blowing agents used may have had particle sizes less than50 micrometers and may have been mixed together to form a powderadditive, there has been no suggestion that the activator and blowingagent should be micronised together. Furthermore, whilst the presence ofan activator may reduce the actual temperature at which decompositionbegins, it will not usually affect the temperature spread over which themajority of the gas is evolved.

Surprisingly, we have now found that if the blowing agent is mixed withthe activator and the mixture itself is then micronised, enhancedresults can be obtained from the mixture. This is most unexpected sincemixing of separately micronised blowing agent and activator does notproduce the same result.

Accordingly, the present invention provides a method for making ablowing agent composition which comprises comminuting a mixture of theblowing agent and an activator for the blowing agent until the meanparticle size of the mixture is less than 5 micrometers.

The invention also provides a blowing agent composition comprising amixture of a blowing agent and an activator for the blowing agentcharacterized in that the mixture has been comminuted to a mean particlesize of less than 5 micrometers.

The invention further provides an expandable polymer compositioncomprising a thermoplastic polymer and a blowing agent composition ofthe invention.

The present invention can be applied to a wide range of thermallydecomposable blowing agents and activators. Thus, suitable blowingagents for present use include nitrogen evolving blowing agents; notablyazo compounds, (e.g. azodicarbonamide) and sulphonhydrazides [e.g.benzene sulphonhydrazide and oxy-bis(benzenesulphonhydrazide)]. Aparticularly preferred blowing agent is azodicarbonamide. Suitableactivators for present use include amines and amides, e.g. containingfrom 1 to 10 carbon atoms; urea; sulphonhydrazides (which may also actas secondary blowing agents); metal salts, oxides or soaps, e.g. oxides,mineral acid salts (e.g. the phosphates, sulphates or carbonates) andfatty acid (e.g. C₈ to C₂₀ aliphatic acid) soaps or complexes of zinc,cadmium, lead or tin.

The activator and blowing agent are present in any suitable amount,typically in weight ratios of 1:20 to 1:1. If desired, other ingredientsmay be present in the mixture before or after micronisation. Suitableother ingredients include secondary blowing agents which may be providedby the use of excess of certain of the activators, e.g. zinc carbonateor a sulphonhydrazide; plasticisers; oils; pigments; lubricants andfillers.

The composition of the invention is made by comminuting the ingredientstogether as opposed to separately as hitherto. The term comminution isused herein to denote the reducing in size of individual particles of amaterial and does not include only the mere breaking up of aggregationsof smaller particles. Comminution can be carried out by any suitablemethod. Preferably the mixture is micronised, notably in a fluid energymill, preferably an air powered mill. Comminution is carried out untilthe mean particle size is less than 5 micrometers, e.g. 2 to 4.5micrometers. The particle sizes quoted herein are those determined bycarrying out a particle size analysis of the material using a CoulterCounter model TA and the mean particle size is that corresponding to a50% retention in the particle size analysis. In an air poweredmicroniser this is achieved by suitable selection of the air pressureand the feed rate of material to the mill.

The blowing agent, activator and other ingredients may be premixedbefore comminution or can be fed separately, e.g. by screw feeds, to thecomminutor.

The blowing agent composition according to the invention may be used forexpanding a wide range of thermoplastic polymers, or polymers which arethermoplastic at the outset of the expansion process but which are crosslinked, vulcanised or otherwise rendered thermoset during or after theexpansion process, for example natural and synthetic rubbers. Suitablepolymers include polyolefins (such as polyethylene, polypropylene andpolybutylene), olefin copolymers (such as copolymers ofethylene/propylene and ethylene/vinyl acetate), polystyrene, styrenecopolymers (such as acrylonitrile/butadiene/styrene copolymers),polyvinyl chloride, and vinyl chloride copolymers. Preferably thepolymer is polyvinyl chloride.

The blowing agent composition is suitably present in the polymer as 0.01to 30, preferably 1 to 5, parts by weight of the active blowing agentingredient per hundred parts by weight of polymer.

Because of its good dispersibility, the blowing agent composition of theinvention can be incorporated directly into the thermoplastic polymerduring the compounding, tumbling or mixing operation using any suitabletechniques. However, if desired, the blowing agent composition can befirst compounded into a pre-mix containing a relatively high proportionof the active blowing agent ingredient, for example 15 to 30 parts byweight per hundred parts by weight of polymer or as much as 70 parts per30 parts of plasticizer when preparing a pre-mix for use with polyvinylchloride. This pre-mix can then be incorporated into the bulk of thepolymer in any of the conventional ways.

The expandable composition can also contain any of the usual additives;for example stabilisers, lubricants, plasticisers, fillers, dyes,pigments and secondary blowing agents.

The blowing agent composition of the invention offers the advantagesover the analogous conventionally blended compositions of a lowerdecomposition temperature, a narrower decomposition temperature rangeand an increase in the rate of evolution of gas from the composition.

The following Examples are given to illustrate the invention, all partsand percentages are by weight.

EXAMPLE 1

200 Parts azodicarbonamide (available under the trade mark GENITRON AC2)were mixed with 60 parts of zinc oxide in a low speed, low shearscrew-mixer.

The above AC/ZnO composition was then micronised in a 12 inch microniserat an air pressure of 85 p.s.i. to give a product with a mean particlesize of 4.5 micrometers. This product is referred to as the AC/ZnOcomicronised composition.

Azodicarbonamide and zinc oxide were micronised separately using thesame technique and the products were then mixed in a high speed mixer togive a blend containing the same proportions as the AC/ZnO comicronisedcomposition. The blended product is referred to as the AC blend below.

The AC/ZnO composition and the AC blend were then heated to causedecomposition of the azodicarbonamide and the amount of gas evolved wasmeasured. The results were as follows:

    ______________________________________                                                      Decomposition                                                                           Total gas evolved                                                   temperature                                                                             cc/g                                                  ______________________________________                                        AC/ZnO   Comicronised                                                                             135° C.                                                                            160                                                    Composition                                                          AC Blend            170° C.                                                                            160                                           ______________________________________                                    

EXAMPLE 2

The process of Example 1 was repeated using 70 parts of AC and 30 partsof zinc carbonate. In this case the comicronised composition decomposedat 136°-147° C. and the blended composition decomposed at 143° to 164°C.

EXAMPLE 3

The following polymer composition was prepared from the AC/ZnOcomicronised composition and AC blend prepared as in Example 1:

    ______________________________________                                        Polyvinyl chloride     100 parts                                              Dioctyl phthalate      57.5 parts                                             Calcium carbonate      15 parts                                               Blowing agent mixture  2.5 parts                                              ______________________________________                                    

The mixture was then coated on to a backing sheet at a rate of 350 g/m²and pre-gelled at 125° C. A transparent non-expanding plastisol layerwas coated onto the expandable layer at a rate of 200 g/m². The productwas heated to 200° C. for 106 seconds to decompose the blowing agent.The results are set out below in arbitrary units of thickness.

    ______________________________________                                                    AC/ZnO Comicronised                                                           Composition   AC Blend                                            ______________________________________                                        Total expanded thickness                                                                    79              67                                              Foam layer thickness                                                                        39              27                                              ______________________________________                                    

EXAMPLE 4

The following polymer compositions were prepared from the AC/ZnCO₃comicronised composition and AC blend prepared as in Example 2:

    ______________________________________                                        Polyvinyl chloride     100 parts                                              Dioctyl phthalate      80 parts                                               Calcium carbonate      5 parts                                                Blowing agent mixture  3.5 parts                                              ______________________________________                                    

The mixture was then coated on to a release paper at a rate of 350 g/m²and pre-gelled at 125° C. The product was heated to 200° C. for timesvarying from 30 to 120 seconds to decompose the blowing agent andproduce a foam. At various time intervals the foam thicknesses weremeasured and the expansion factors calculated. The expansion factor isthe thickness of the foam divided by the original unexpanded thickness.The expansion factors were plotted against time and the times taken toachieve given factor values established. The results were as follows.

    ______________________________________                                                                 % reduction in dwell                                 Expansion Oven Dwell Time                                                                              time using comicronised                              Factor    Comicronised                                                                             Blend   composition                                      ______________________________________                                        2.0       35         50      30                                               2.5       39         60      35                                               2.8       45         70      35                                               ______________________________________                                    

    __________________________________________________________________________           Expansion           Oven Dwell Time                                                                          Oven Temperature                                                                        % Reduction                   Example No                                                                           Factor                                                                              Blowing Agent Composition                                                                   Comicronised                                                                         Blend                                                                             °C.                                                                              in dwell                      __________________________________________________________________________                                                    time                          5      2.0   70                                                                              parts AC    42.5   51  200       16                                   2.5   30                                                                              parts ZnO   46     57.5                                                                              200       20                                   2.8                 49     62.5                                                                              200       21                            6      2.0   70                                                                              parts AC    45     52.5                                                                              200       14                                   2.5   30                                                                              Zn stearate 50     72.5                                                                              200       31                                   2.8                 56     86  200       35                            7      2.0   60                                                                              parts AC    63     97  170       35                                   2.5   25                                                                              parts ZnO   79     131 170       40                                   2.8   15                                                                              parts urea  89     152 170       41                            8      2.0   95                                                                              parts AC    100    135 190       26                                   2.5   15                                                                              parts benzene sulphon-                                                                    135    200 190       32                                           hydrazide                                                                   0.1                                                                             parts of ZnO per 100                                                          parts of resin were                                                           incorporated into the                                                         polymer                                                        __________________________________________________________________________

I claim:
 1. A method for making a blowing agent composition whichcomprises a solid, thermally decomposable blowing agent and a solidactivator for the blowing agent characterised in that a mixture of theblowing agent and the activator is comminuted until the mean particlesize of the mixture is less than 5 micrometers.
 2. A blowing agentcomposition comprising a mixture of a solid, thermally decomposableblowing agent and a solid activator for the blowing agent characterisedin that the mixture has been comminuted to a mean particle size of lessthan 5 micrometers.
 3. A method as claimed in claim 1 wherein theblowing agent is selected from an azo compound or a sulphonhydrazide. 4.A method as claimed in claim 1 wherein the activator for the blowingagent is selected from amines and amides, urea; sulphonhydrazides; metalsalts, oxides or soaps.
 5. A method as claimed in claim 1 wherein theactivator is used in weight ratios of 1:20 to 1:1 based on the weight ofthe blowing agent.
 6. A method as claimed in claim 3 wherein said azocompound is azodicarbonamide.
 7. A method as claimed in claim 4 whereinsaid amines and amides have alkyl groups containing from 1 to 10 carbonatoms.
 8. A method as claimed in claim 4 wherein said metal salts andoxides are oxides or salts of zinc, cadmium, lead or tin.
 9. Acomposition as claimed in claim 2 wherein the blowing agent is selectedfrom an azo compound or a sulphonhydrazide.
 10. A composition as claimedin claim 9 wherein said azo compound is azodicarbonamide.
 11. Acomposition as claimed in claim 2 wherein the activator for the blowingagent is selected from amines and amides, urea, sulphonhydrazides, metalsalts, oxides or soaps.
 12. A composition as claimed in claim 2 whereinthe activator is used in weight ratios of 1:20 to 1:1 based on theweight of the blowing agent.
 13. A composition as claimed in claim 11wherein said amines and amides have alkyl groups containing from 1 to 10carbon atoms.
 14. A composition as claimed in claim 11 wherein saidmetal salts and oxides are oxides or salts of zinc, cadmium, lead ortin.